368 



MR. C. REID ON THE ORIGIN OF DRY CHALK 



being eroded. In fact the descent of Coombes near Pottingdean to 

 the sea-level, and the way that other Coombes plunge beneath the 

 tidal marshes of the Adur and Aran, are strongly suggestive of a 

 slight elevation while the erosion was taking place. 



Prof. Prestwieh's view that the deep trenching of the Downs by 

 valleys has gradually lowered the plane of saturation and dried the 

 Coombes, is clearly applicable to a large number of the slightly 

 inclined Coombes like those through which the two lines of rail 

 pass near Brighton. But the whole structure of the country, the 

 proximity of the escarpment on the north, of the old sea-cliff on the 

 south, and of the deep valleys of the Wealden rivers, which traverse 

 the Downs from north to south, shows clearly that the outlet for 

 the water must have been just as free then as now, so far as deep 

 trenching could aid it. All these features existed before most of the 

 Coombes were cut or the Coombe Pock deposited. The greater depth 

 to which the Coombes have now been cut can be left out of account ; 

 for dry Chalk valleys play no part in the present superficial drainage, 

 and it would make little, if any, difference in the height of the plane 

 of saturation if they were filled up again. 



The trenching cannot therefore affect the steeper Coombes, where 

 the water-level is hundreds of feet below the surface and the incline 

 very great. The slope of the plane of saturation in Chalk never 

 exceeds, if it reaches, 60 feet in a mile ; here we are dealing with 

 slopes seldom less than 100 feet, and in several instances reaching 

 500 feet in the mile. To illustrate this, I have drawn on a true scale 

 sections of the three Coombes in the South Downs, which are most 

 generally visited (fig. 2). 



No change could cause springs to break out in the upper portion 

 of these valleys, unless it were a deep submergence of the whole of 

 the adjoining low lands. If such upland springs had formerly ex- 

 isted, their successive failure through the gradual lowering of the 

 plane of saturation would necessarily have left evidence in extensive 

 deposits of rolled gravel. Winding terraces would also border the 

 Coombes at various heights, like the terraces round the Scottish or 

 Norwegian fiords. The low lands ought also to yield abundant evi- 

 dence of a submergence of 100 fathoms, whether under sea or 

 fresh water. 



Nothing of the sort occurs, and one of the most striking charac- 

 teristics of these dry Chalk valleys is the almost entire absence in 

 them of gravel and the absence also of definite terraces. If terraces 

 formerly existed they ought still to be found ; for when once left 

 dry they would be out of reach of running water, and might be 

 preserved for an indefinite time. 



There is another difficulty which prevents us from accepting either 

 of the above-mentioned theories. Nearly all the valleys traverse 

 the whole breadth of the Downs, and then end abruptly just before 

 the escarpment is reached. If these valleys had been gradually Cut 

 back by springs, many of them ought to fall northward to the 

 escarpment, where most of the large springs are found ; but nearly 

 all the Chalk Coombes follow the general slope of the ground and 

 open to the south. 



